You have to keep in mind that the SL is slightly directional while the Proto is a true twin. Also, the SL is more damp than the Proto. This might not be ideal for learning jibs (boxes and rails). The Proto has a nice dampness to it and you can feel what's underfoot. However, this means the Proto doesn't perform as well on the mountain as the SL.
This comes down to exactly what you plan on doing. If you are going to focus on park more, I will say the Proto is a better choice. If you want to continue mainly freeriding, the SL will be better.
As for size, the 154 Proto will ride like a 156 because it has blunted tips. Just think of it as a 156cm board. I'd do the 155 in the SL since you plan on mixing park in your riding.